[PD] GEM GEMglClearColor alpha ?

james tittle tigital at mac.com
Tue Sep 13 17:12:59 CEST 2005

On Sep 13, 2005, at 1:19 AM, vade wrote:

> Hello


> Playing with the gl objects in GEM (0.90) on OS X (Pd 0.38-4  
> Extended, via Hans) , ive noticed that GEMglClearColor has 5  
> inputs, one for the gem render chain, 2 - r, 3 - g, 4 - b and  
> presumably 5 - alpha, yet, changing alpha results in no change in  
> the window Changing the RGB values does, just like [color] to  
> gemwin (as it should).
> Presumably, changing the alpha would act to let the scene draw  
> 'over' itself, and give an easy way to do trails/blurring. Jitter  
> supports this feature (not griping.. just saying :), any chance ,  
> or workaround to get similar functionality, as I love the aesthetic  
> of it. Id rather not use snap2pix/snap2tex.

...ok, you have the correct inputs to the object, but there are a  
coupla things to make sure of:

1.  the rgba inputs need to be 0-1
2.  after [GEMglClearColor] you need a [GEMglClear] that clears the  
color buffer (GL_COLOR)
3.  what is the gemhead ordering?  Are you trying to clear everything  
via [gemhead 1]?  Or are you inserting this into every drawn object's  

...that's the theory, anyway :-)  In practice in GEM, things are not  
exactly as they may appear...in other words, while I think the  
glWrapper is one of GEM's most powerful features, there are a few  
decisions made as default behaviors that may cause unexpected  
behavior and frustration...fr'instance, glClearColor is called many  
times behind your back, such as windowInit, window reset, and the  
color message to [gemwin]...

...hmm, now going thru this, you may just want to play with that  
message to [gemwin] in conjunction with the clearmask message, which  
I think you'll need to send a [GL_COLOR< - [GLdefine] to...?  I'll  
have to try that out when I have GEM running...

...anyway, as ya can see, there's a bit more to the glWrappers than  
just trying an object...it's really important to have a copy of the  
"Red Book" on hand, but in general I've found that it's an extremely  
quick and powerful way to prototype:  no more compiling ;-)


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